DURHAM, N.H., — The Portsmouth woman who pleaded guilty Thursday to lying to police about Elizabeth Marriott's murder could be a valuable asset to prosecutors if her ex-boyfriend goes to trial on murder charges next year.

Kathryn McDonough, 19, was sentenced on charges of conspiracy, witness tampering and hindering prosecution. She was ordered to serve three consecutive jail terms of between 3½ and 7 years — a potential incarceration of up to 21 years. But under the terms of a negotiated plea deal, a majority of that time was suspended, and McDonough will serve 1½ to 3 years in prison. The deal requires McDonough to testify in the upcoming murder trial of her former boyfriend, Seth Mazzaglia, who is accused of strangling Marriott, of Westboro, Mass., inside his apartment on Oct. 9, 2012.

One of the most significant challenges in any criminal trial is proving the suspect's intent, according to Charles Putnam, clinical associate professor of justice studies at the University of New Hampshire.

People don't often spell out their rationale before they act, Putnam said, but some clues can help shade the jury's understanding of someone's state of mind: What did the suspect do and say before the crime? What did he look like while it was going on?

In Mazzaglia's case, prosecutors have laid out three theories of the murder. One first-degree murder count alleges Mazzaglia "purposely" strangled Marriott to death with a rope. Under state law, "purposely'' is defined as meaning the actor's conscious object is the death of another, and that the killing was deliberate and premeditated.

Another first-degree murder charge asserts Mazzaglia "knowingly" caused Marriott's death in the course of a sexual assault. He was also indicted on a second-degree murder charge, which alleges he "recklessly" caused Marriott's death with extreme indifference to the value of human life.

If the testimony of a cooperating co-defendant helps establish the elements of first-degree murder, it can have tremendous consequences in sentencing. First-degree murder carries a mandatory life sentence without the possibility of parole in New Hampshire. A second-degree murder conviction still carries the potential for a life sentence, but it isn't mandatory.

In most criminal cases, criminal defense attorneys and prosecutors begin to discuss whether the case is eligible for plea negotiation almost as soon as the charges are filed, said Putnam, who is co-director of UNH's Justiceworks program.

Defendants aren't required to take the stand when they are tried in court, and in most instances, witness testimony is likely to be persuasive to the jury. That's especially true if it corroborates the physical evidence or statements of other witnesses.

"Very often a homicide occurs without witnesses, and if there is an eyewitness to the homicide, that eyewitness is very likely to have testimony that will be very helpful to a jury to understand the details of what occurred in the case," Putnam said.

Prosecutors prefer to have the least culpable of multiple defendants testifying on the state's behalf, because defense attorneys will be eager to cross-examine a cooperating co-defendant.

Mazzaglia is accused of using a rope to strangle Marriott, then discarding her body in the waters off Peirce Island in Portsmouth. He initially denied any involvement in Marriott's death, but later confessed to strangling the UNH sophomore under questioning, according to a police affidavit.

Mazzaglia was indicted on murder charges by a grand jury in May. He is also accused of conspiring with McDonough to deceive police. He has entered not guilty pleas on all charges. Mazzaglia's trial is tentatively scheduled for May 2014. The trial is expected to take a week and a half, plus another week for jury selection.